If I have a correct ID on this bird, it's a Northern Harrier, the only North American representative if the Harrier group. It's characteristic hunting pattern is to fly low over Marshes or meadows, ready to pouch on small animals who may be out in the open. It is known as the Hen Harrier in Europe.
The author is less than totally confident of the ID on this bird.
Additional information: see Wikpedia - Hen Harrier